Trimellitic acid (TMA, 1,2,4-benzenetricarboxylic acid) is an important product which finds application as an intermediate in the chemical industry. The most important reaction of tri-mellitic acid is its dehydration to trimellitic anhydride, which in turn is used as a starting material for the production of polymers and chemical intermediates.
Applications of trimellitic anhydride include resins for powder coatings, inks, wire enamels, high performance plasticizers with low volatility, and engineering polymers for high temperature applications.
Trimellitic acid is commercially produced by oxidation of pseudocumene (1,2,4-trimethylbenzene).
All commercial processes oxidize pseudocumene in the liquid phase, in the presence of acetic acid as a solvent and of a catalyst which includes as basic components cobalt, manganese and bromine.
This is the so called Mid Century Catalyst, which was developed in the 1950's and which is applied by most commercial processes for the oxidation of alkyl aromatics (W. Parteheimer, Catalysis Today, 1995, 69-158). Terephthalic acid and isophthalic acid are produced according to this technology by liquid phase oxidation of p-xylene and m-xylene, respectively, in the presence of acetic acid as a solvent and of a catalytic system including cobalt, manganese and bromine. In the case of terephthalic acid and isophthalic acid the oxidation reaction is performed in a continuous way. Air and the mixture of solvent, reagent and catalyst are fed continuously to the reactor which operates under pressure and at temperatures in the range of 150 to 200° C. Under these conditions the reagents are oxidized to the corresponding polycarboxylic acids and the reaction mixture is continuously withdrawn from the reactor.
The oxidation of pseudocumene to trimellitic acid is more difficult than the oxidation of p-xylene and m-xylene due to the presence of three methyl groups on the aromatic ring. For this reason a more active catalyst is required in order to obtain acceptable yields and a viable crude quality: in the commercial oxidation of pseudocumene, additional metals, beside manganese and cobalt, are used to increase the activity of the catalyst.
Trimellitic acid, which is the desired product of the reaction, is a poison for the catalyst. For this reason the commercial oxidation of pseudocumene to trimellitic acid is performed batch-wise in order to reach high concentrations of trimellitic acid only in the final stage of the reaction. In the batch process, acetic acid, pseudocumene and catalyst are loaded into the reactor, which is heated up and pressurized. After reaching the desired reaction pressure and temperature, air is fed to the reactor till the oxidation of pseudocumene is completed.
The main drawback of the batch process is that the concentration of the reactant pseudocumene is very high at the beginning of the reaction, when the oxidation is very easy (the oxidation of the first methyl group is easier than the oxidation of the second and the oxidation of this is easier than the oxidation of the third group). In the initial stage the oxidation is difficult to control and the low concentration of dissolved oxygen favors undesired reactions which produce high boiling by-products, and dealkylation and transalkylation reactions which lead to the formation of phthalic acids, in particular isophthalic and terephthalic acid, and pyromellitic acid.
To partially overcome these problems, prior art patents teach the use of a temperature gradient during the batch process: a lower temperature in the first part of the oxidation can prevent the oxygen starvation but cannot avoid the presence of high concentrations of pseudocumene in the first stage of the reaction.
Since the oxidation becomes more difficult going from the first to the second to the third methyl group and since the final oxidation product trimellitic acid is a poison for the catalyst, some patents teach the feed of additional catalyst during the course of the batch oxidation in order to maintain and/or increase the activity of the catalytic system.
U.S. Pat. No. 3,683,016 A describes a batch oxidation of pseudocumene in the presence of a catalyst including cobalt, manganese, cerium and bromine. The first part of the reaction is performed at a lower temperature and part of the catalyst is added stepwise during the batch oxidation of pseudocumene to trimellitic acid.
The use of a temperature gradient and the staged addition of the catalyst is also taught by U.S. Pat. No. 5,250,724 A which uses an improved catalyst including cobalt, manganese, cerium, titanium and bromine.
U.S. Pat. No. 4,755,622 A uses cobalt, manganese, zirconium and bromine as catalyst while the catalyst of U.S. Pat. No. 4,992,579 A contains cobalt, manganese, cerium, zirconium and bromine. These two patents mention, in addition to the temperature gradient and to the staged addition of the catalyst, the possibility of performing the first part of the oxidation in a semi-continuous way: instead of adding all the pseudocumene to the initial reaction mixture, it is gradually fed to the reactor in the first stage of the oxidation. The staged addition of the catalyst foresees the addition of max 35% of total bromine in the first stage, the catalyst added in the second stage includes the total cerium used in the process and part of manganese and zirconium: the compositions of the initial catalyst and of the catalyst added during the reaction are different. The first part of the oxidation, which can be performed in semi-continuous way, is conducted so that the theoretical oxygen uptake is between 1 and 2.5 mol of oxygen/mol hydrocarbon and more preferably between 1.5 and 2 mol of oxygen/mol hydrocarbon. This corresponds to a consumption of oxygen in the semi-continuous stage between 22 and 56% of the theoretical for the preferred condition and between 33 and 44% for the most preferred one.
U.S. Pat. No. 4,992,579 A does not describe the semi-continuous oxidation in the examples.
U.S. Pat. No. 4,755,622 A describes the semi-continuous stage in examples 11 and 13 (both with catalyst staging) and in comparative examples F and H (no catalyst staging). The yield of isophthalic acid plus terephthalic acid (undesired bifunctional byproducts) in most examples is around or over 2 mol %.
In spite of the improvements the quality of the crude trimellitic acid is still not completely satisfactory due to the presence of several byproducts, in particular isophthalic acid and terephthalic acid. A low quality of the crude trimellitic acid is reflected in the final quality of tri-mellitic anhydride and makes the purification of trimellitic anhydride questionable and more difficult. The presence of dibasic acids like terephthalic and isophthalic acid in particular is a main drawback because trimellitic anhydride is used in demanding applications where the presence of bi-functional molecules is detrimental.
The feeding of the catalyst during the batch (second) stage of the oxidation reaction is a complication for the process and leads to an increase of the catalyst consumption. Moreover, when a staged catalyst addition is applied, the composition of the initial catalyst and that of the catalyst added during the reaction are different, this makes impossible the recovery and the recycle of the catalyst contained in the final reaction mixture.
Even with the improvements mentioned in the existing patents, the present processes for the production of trimellitic acid are not completely satisfactory.